Carbon oxide conversion processes are of considerable importance in the manipulation of synthesis gas by the water-gas shift reaction and the production of alcohols such as methanol. These reactions are depicted below.CO+H2O→CO2+H2 CO+2H2→CH3OHCO2+3H2→CH3OH+H2O
The catalysts for such reactions are generally produced by forming into pellets small discrete particles of an intimate mixture of copper oxide and one or more other oxidic materials, generally including zinc oxide, that are not reduced under the conversion reaction process conditions. The intimate mixture is generally made by precipitation of copper compounds and compounds convertible to the other oxidic materials, and/or precipitation of the copper compounds in the presence of the other oxidic materials or compounds convertible thereto, followed by calcination to convert the precipitated copper compounds, and other components as necessary, to the oxides. Hence pellets are formed form oxidic powders. In order to generate the active catalyst, the pellets are subjected to reducing conditions to reduce the copper oxide in said pellets to metallic copper. The reduction step is normally carried out in the reactor where the carbon oxide conversion process is to be effected: thus normally a catalyst precursor in which the copper is present in the form of copper oxide is charged to the reactor and the reduction effected by passing a suitable gas mixture there-through. The reduction of copper oxide is exothermic and the in-situ reduction step is often carried out over extended periods using dilute hydrogen streams to avoid damaging the catalyst. Such extended start-up procedures are difficult to control and can be costly to operate.
By such precipitation/calcination/reduction techniques, the catalysts generally have a copper surface area above 20 m2 per gram of copper, often above 40 m2 per gram of copper. Commercially available carbon oxide conversion catalysts typically have a copper surface area about 50 m2/g per gram of copper. Copper surface area may be measured by the nitrous oxide decomposition method, e.g. as described in the article by Evans et al. in Applied Catalysis 1983, 7, 75-83 and a particularly suitable technique is described in EP 0202824.
Since the activity of the catalysts is linked to the copper surface area, it is desirable to obtain catalysts with higher copper surface areas.
U.S. Pat. No. 4,863,894 describes a process for the manufacture of a catalyst comprising forming a composition comprising an intimate mixture of discrete particles of compounds of copper, and zinc and/or magnesium and, optionally, at least one element X selected from aluminium, vanadium, chromium, titanium, zirconium, thorium, uranium, molybdenum, tungsten, manganese, silicon, and the rare earths, and subjecting the composition to reduction conditions so that the copper compounds therein are converted to copper, wherein the copper compounds in the intimate mixture are reduced to metallic copper without heating said intimate mixture to a temperature above 250 DEG C. The direct reduction of the precipitated catalyst precursor compositions, rendered catalysts having copper surface areas >70 m2 per gram copper.
However copper surface area is not the only criterion that needs to be taken into account for carbon oxides conversion catalysts. In particular catalyst strength and stability, both in terms of activity and selectivity, are also important. Mean Horizontal Crush Strength (MHCS) is a method widely used in the catalyst industry to measure the strength of catalyst pellets. MHCS is routinely measured on pellets to ensure their strength is sufficient to undergo the stresses applied during catalyst loading and to give an indication of strength in duty. The catalysts obtained by the process of U.S. Pat. No. 4,863,894 do not have the high strength stability required in modern carbon oxides conversion processes, and currently oxidic catalysts are still used.
We have now devised catalysts of increased copper surface area that overcomes the disadvantages of the previous catalysts.